Posted before, but I'll bet I'm not the only one who has
forgotten it.
Date: Mon Oct 14 00:01:06 EDT 2002
Subject: A.Word.A.Day--hapax legomenon
hapax legomenon (HAY-paks li-GOM-uh-non) noun, plural hapax legomena

A word or form that has only one recorded use.

[From Greek hapax (once) + legomenon, from legein (to say).]